1. Field of the Invention
The invention relates to digital communication generally and to digital communication using infrared radiation in particular.
2. Description of the Prior Art
Infrared radiation has long been used for portable communications sytems designed for interior use. A simple example is the portable control device used to control consumer electronic devices such as televisions or VCRs. Most such control devices use infrared light to carry control signals from the device to the television or VCR.
Use of infrared radiation for digital communications in the interiors of buildings is also attractive. The infrared radiation has no noxious side effects and systems in different rooms will not interfere with each other. A problem with the use of infrared radiation for digital communications is, however, the fact that the typical interior environment produces a great deal of infrared noise. All types of AC lighting produce infrared noise and fluorescent lighting fixtures are particularly noisy. Two techniques have been used in the past to overcome the noise:
in one approach, the power of the infrared transmitter has simply been increased as required to overcome the noise and PA1 in the other, the signal employed by the communicating devices has been modulated up so that its frequency is different from that of the noise.
Both of these approaches have disadvantages. The first approach requires high levels of power; the second approach requires extra circuitry to do the modulation and demodulation of the infrared signals and infrared light emitting diodes capable of operating at the rate required for the transmission frequency. These disadvantages are particularly important in the area of portable digital communications equipment. The need for high power in portable equipment means that the equipment must be expensive, heavy, or both; the need for extra circuitry and fast light-emitting diodes adds to the expense. What is provided by the present invention is a simple, low-cost, low-power technique for distinguishing an infrared signal from infrared noise.